That image is of the galaxy cluster CL0024+1652 (go look at the higher resolution version — it’s very pretty!), a galactic city located a whopping 5 billion light years away! That means the light we see from this cluster left it five billion years ago, so we’re seeing this structure as it was when the Universe was just 2/3 its present age. Almost every small object in that image is a galaxy, and all of them are held sway by the cluster’s gravity, orbiting the center like bees flying around a beehive.

It has long been thought that every large object in the Universe is surrounded by a halo of dark matter — unseen, mysterious, yet profoundly influential in the life of normal matter. Dark matter (or just DM for short) gives off no light, and does not interact with normal matter directly– a cloud of it could pass right through you and you’d never know. But, like regular old matter, it has gravity, and that can betray its presence.